1. Field of the Invention
The present invention relates to image processing method and apparatus which correct a distribution of brightness of an image by an image process.
2. Related Background Art
Conventionally, as a method of taking a photograph of appropriate brightness, a method of measuring average luminance of a scene to be taken or photographed and then controlling shutter speed, a aperture value and the like based on the measured average luminance is known. Moreover, an exposure control method based on so-called evaluation photometry.,that a scene is first divided into predetermined areas, luminance of each of the divided areas is measured, the measured luminance is appropriately weighted with respect to each area, average luminance is then obtained based on the weighted values, and appropriate exposure is thus obtained with respect to the entire scene is known.
Incidentally, in a scene taken against light (also called a backlight scene) that the brightness of a main subject is remarkably dark as compared with that of its background, the main subject portion on a taken or photographed image is inevitably dark. Therefore, to take a photograph in which appropriate brightness can be secured even in case of a backlight scene, it is necessary at the moment of taking the photograph to preset exposure of a camera so that the subject is taken brightly as compared with a case of taking an average photograph. However, such an exposure correction operation is troublesome for a user, and moreover skill is required to appropriately set the exposure of the camera. Besides, even if the exposure correction is appropriately performed with respect to the main subject, the background portion of the main subject adversely tends to become bright excessively.
In analog photographing technique, print of appropriate brightness can be obtained by performing so-called a dodging process in a darkroom. Therefore, it is desirable even in a digital image process to easily achieve a dodging process similar to that in the analog photographing technique.
For example, a method of achieving the dodging process in the digital image process is proposed by Daniel J. Jobson et al. in “A Multiscale Retinex for Bridging the Gap Between Color Images and the Human Observation of Scenes”, IEEE TRANSACTIONS ON IMAGE PROCESSING, VOL. 6, NO. 7, JULY 1997 (hereinafter called a prior art 1). In this method, a difference process between a component obtained by logarithmic transforming a digital image and a low-frequency component of the logarithmic-transformed component is performed to darken a bright component and brighten a dark component in a low-frequency area in the digital image, thereby achieving image improvement.
Besides, another method of achieving the dodging process in the digital image process is proposed by Reinhard et al. in “Photographic Tone Reproduction for Digital Images”, acm Transactions on Graphics, JULY 2002, Vol. 21, No. 3 (hereinafter called a prior art 2). In this method, a dodging-like effect is obtained in the digital image process by using a luminance component of a digital image and a low-frequency component thereof.
However, in such conventional methods, when a noise characteristic of the digital image to be processed is deficient, a noise on the image is emphasized by the above image improvement process and thus becomes noticeable if anything. Moreover, when the number of gradations is small, a so-called gradation jump becomes noticeable. Furthermore, when contrast and chroma of the digital image are high, the contrast and the chroma are excessively emphasized by the above image improvement process, whereby a satisfactory image cannot be obtained.